In the semiconductor industry, the transferring of process chemicals, particularly high purity process chemicals, from bulk shipping containers to the process areas is a very critical and often a dangerous operation in the manufacturing of semiconductor integrated circuits. The simplest and initially most common method for transferring process chemicals was pouring the chemical from bulk containers, usually limited to one and five gallon containers. However, this method limits the size and weight of the containers, requires considerable labor and is dangerous when hazardous chemicals are involved. Another early attempt in the transfer of process chemicals was the utilization of conventional pumping and transfer devices, such as impeller or centrifugal pumps. However, these devices proved to be unsatisfactory, primarily due to the corrosive nature of the process chemicals and the need for high purity standards.
More recently, the industry has begun to develop pumping systems for use with larger and more cost effective bulk containers, typically the standard 55 gallon drum. Considerable effort has been expended in the development and refinement of semiconductor chemical pumping and delivery systems. Currently, there are primarily three basic systems being used by the industry. The first system utilizes pneumatically driven positive displacement diaphragm or bellows pumps which pump the chemical from the drum either directly to the desired process area or to a storage tank before being pumped to the process area. Although these pumps are commercially available, they are relatively expensive due the chemically resistant materials required in their construction. In addition, they require high maintenance, especially when used for pumping wafer polishing slurries, due to the corrosive and abrasive nature of these chemicals. The second system involves a pressurized dispense system wherein chemical first is pumped into a pressurized liquid tank and then is pressure dispensed from the pressurized tank to the desired process area. Currently, these systems are custom built and the pressurized tank containing the chemical require an internal chemical resistant lining. The third system relies on vacuum/pressure technology utilizing a minimum of two vacuum/pressure chambers. This is a hybrid system that necessitates the use of numerous valves and sensors, as well as extensive plumbing and controls.
The primary performance limitation of each of these pumping systems is their inability to quickly extract the chemical from the drum and to the pressure side of the pump as fast as the positive side of the pump is capable of pumping it. In other words, there are "dry lift" or self-priming limitations associated with these pumping systems because the force that creates the input pressure in these pumping systems is primarily the result of atmospheric pressure. The maximum pressure on the inlet side will always have the limitation of 14.7 psi. This problem becomes even more evident if the chemical has a high specific gravity or viscosity, or both. This problem also is true for some of the semiconductor process chemicals, especially sulfuric and phosphoric acid.
In addition to the "dry lift" problem that is inherent in the design of these chemical transfer systems, there are additional intrinsic deficiencies in these systems which become extremely critical and significant in the semiconductor industry where ultra high purity process chemicals are used almost exclusively. One such deficiency is the generation of particle contamination created by the pumping mechanisms themselves. The positive displacement pumps employed in the first two systems described above are a source of particle contamination due to the rapid flexing of the diaphragm or bellows in the pump. This continuous flexing of the diaphragm or bellows material causes mechanical degradation of the component elements and results in the release of particles into the fluid stream. An additional source of particle contamination is derived from the check valves used in these pumps; the check valves cycle at the same rate as the flexing diaphragm or bellows, and due to the abrading nature of the check valve, release particles into the fluid stream. The third above-described system of transferring chemical, which utilizes pressure and vacuum, has resulted in a reduction of some of the particle contamination problem by the elimination of the bellows of diaphragms from the pump. However, the third system still incorporates valves that open and close continuously, and as with the check valves discussed above, the same abrading problems exist that create particle contamination in the fluid stream.
A second deficiency in these systems is their inability to maintain smooth and constant flow across sensitive ultrapure filtration media utilized in the semiconductor industry. These specially designed filter membranes have a pore size filtration capability as small as 0.1 .mu.m and are very delicate and quite expensive. Further, the filtration performance of such filters is very sensitive to fluctuations across the filter membrane. Since positive displacement pumps have extreme pressure and flow pulsating problems, they are detrimental to the filtering performance of these ultrapure filers. Surge suppressors of various designs have been developed to alleviate the problems associated with the positive displacement pumps, but do no eliminate entirely the pulsing. In addition, these surge suppressors add complexity and cost to the pumping system. The second and third prior art systems were developed primarily to resolve this flow pulsation problem. Both utilize a pressurized liquid vessel instead of a positive displacement pump to smooth the flow for filtering and final delivery. However, due to the changing level of the liquid in the vessel as the chemical is transferred, the head pressure of the liquid at the outlet is constantly changing, thereby effecting the flow across the filtering media.
A third inherent deficiency is found in the pressure/vacuum system described above which uses pressure and vacuum to transfer chemicals, this problem is associated with outgassing or boiling off of some of the liquid when the vacuum is applied to the chemical while filling its vessels. This especially is true for some of the more volatile chemicals such as alcohol and other solvent based chemicals. In addition, systems of this type, when used to continually circulate blends of chemicals such as micro-abrasive slurries used for wafer polishing, can affect the balance of the blend and suspended solids content due to higher volatile chemicals boiling off over time while leaving the solids and other chemicals behind.
Moreover, none of the prior art systems have the ability to monitor and adjust both chemical flow and pressure fluctuations in the distribution or output line. Maintaining flow and pressure is important in the output line particurlarly for micro abrasive polishing slurries used for chemical mechanical planarization (CMP). In such processes, a specific flow must be maintained in the distribution plumbing and lines in order to prevent abrasive solid particulates from settling out of suspension and accumulating in the plumbing. These slurry particles can harded in the plumbing if flow is not maintained. In addition to the flow requirements, the pressure in the distribution lines often needs to be maintained because the final dispensing of the slurry can be a timed event that requires a specific pressure to achieve a specified volume requirement at the point of use, such as the process area.
Despite the efforts of the prior art, a need still exists for a chemical dispensing system that dispenses chemical directly from a container/drum and is capable of maintaining chemical flow and/or pressure requirements within the output lines and overall plumbing system. Such a chemical dispensing system should not depend on the use of pumps or pumping systems which extract a chemical from a container/drum to a process area. In addition, such a system should minimize or eliminate inherent "dry lift" and particle contamination problems associated with pumping systems. Such a dispensing system should provide high flow capability with a pulseless and constant flow of liquid chemical directly from a standard drum. In addition, such a dispensing system also should maximize and optimize the performance of state of the art filtering media, particularly ultrapure filtration devices. Moreover, such a system should not subject the chemical to be dispensed to low pressures or vacuum, thereby reducing or eliminating outgassing, boiling off of volatile vapor or precipitation of micro-bubbles in the chemical. Finally, such a chemical dispensing system should provide means for quickly and easily regulating either the pressure in the distribution line to the chemical flow from the drum to the process area and/or provide means for quickly and easily controlling a chemical output valve disposed downstream of the container/drum.